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One of the etymologies for the name Vättern is from "vatten", the Swedish word for water. This origin is, however, unclear and in dispute. It has also been suggested that the archaic term "vätter", meaning forest or lake spirits, is the origin of the lake's name.

The lake's total surface area is about 1,912 km2 (738 sq mi), with a drainage basin a little over double that, about 4,503 km2 (1,739 sq mi). The deepest known point, located to the south of the island of Visingsö, is 128 meters (420 ft). The average depth is 41 meters (135 ft). The lake has a perimeter of about 642 km (399 mi). The volume is 77.0 km3 (18.5 cu mi). These numbers tend to be fixed, as the level of the lake is regulated.

In the north there is a scenic but not mountainous inland fjord, Alsen. About 62% of the drainage basin is still covered with spruce, pine and deciduous forest. About 26.7% is dedicated to agriculture.

The geology of the lake is closely bound to that of the Baltic Sea, of which it was often part. The lake, with Vänern, was part of a connecting waterway system through central Sweden to the Skagerrak several times in the recent geological past.

Vättern is located in a graben that was formed by crustal movements in the east-west direction 40 to 50 Mya.[2] During the most recent millions of years multiple glaciations have covered the lake and its surroundings, leaving glacial striations and drumlins as they receded.

The present-day lake began as an independent body of water left by the receding Scandinavian glacier after the last glacial period around 10,000 BP. It became a minor bay of the Baltic ice lake. Most of the lake's relict species (like the Arctic char) date from that time. Subsequently, it was a bay of Yoldia Sea and then became connected to Ancylus Lake, discharging from the north end of its extent. At about 8000 BP an accident of the uneven Scandinavian isostatic land rise brought Vättern above Ancylus and the two became distinct.

The annual post-glacial rebound today is 3.5 mm (0.14 in) in northeastern Motala and 2.6 mm (0.10 in) in southern Jönköping. This means that Vättern is tilting to the south by 1 mm (0.039 in) every year.[2]

Vättern has been famous for the excellent quality of its transparent water. Many of the municipalities in the area receive their drinking water directly from Vättern. The lake water requires very little treatment before being pumped into the municipal systems and the natural, untreated water can be safely drunk from almost any point in the lake. It has been suggested that Vättern is the largest body of potable water in the world. The surrounding municipalities process 100% of their sewage.

Vättern is known for the annual recreational cycling race Vätternrundan, attracting some 20,000 participants to finish the 300 km trip around the shores of the lake.

Vättern is also noted for its fishing, serving people in the nearby districts. Tourist sport fishermen and vacationers are free to fish in the lake as long as they don't use nets. The lake is also used for commercial fishing.

Admirable, above the rest, are the incomprehensible wonders of the bottomless Lake Vether, over which no fowl flies but is frozen to death, nor any man passeth but he is senselessly benumbed like a statue of marble.

All the inhabitants round about it are deafened with the hideous roaring of his waters when the winter breaketh up, and the ice in his dissolving gives a terrible crack like to thunder, whenas out of the midst of it, as out of Mont-Gibell, a sulphureous stinking smoke issues, that wellnigh poisons the whole country.